Photoelectric system



' March 31 1936.

A. J. MCMASTER 'ET AL 2,035,907

PHOTOELECTRIC SYSTEM Filed Dec. 10, 1932 A/F/Y/e J McMasfer Jaws Q k e//s 53/ My 7 777 A7250.

Patented Mar. 31, 1936 UNITED STATES PATENT OFFICE PHOTOELECTRIC SYSTEM Illinois Application December 10, 1932, Serial No. 646,630

8 Claims.

Cur present invention relates generally to electronic devices and more particularly to photoelectric relay systems.

Photoelectric relay systems are extensively used in industrial applications for counting, inspection, process control and the like.

An object of our present invention is the provision of an improved photoelectric relay.

A further object is the provision of an improved 1o electronic amplifier system.

A further object is the provision of a photoelectric relay of high sensitivity.

A still further object is the provision of a photoelectric amplifier system in which the photoelectric tube is directly coupled to its amplifier valve so as to avoid the use of coupling impedance devices.

Another object is the provision of a relay that is simple in construction and economical to 20 manufacture.

Other objects and advantages will appear as the description proceeds.

According to our present invention we provide a photoelectric system in which a photoelectric 25 tube is directly coupled to the control grid of a thermionic valve without the use of any auxiliary coupling devices. The tube is preferably connected between the control grid and a source of bias control voltage, the anode of the tube being 30 connected to the grid.

In order to acquaint those skilled in the art with the teachings and practice of our invention, we shall now describe a specific embodiment thereof, reference being had to the accompanying drawing forming a part of this specification and in which,

Fig. 1 illustrates diagrammatically, one form of the preferred embodiment of our invention;

Fig. 2 is a chart illustrating graphically certain 40 typical characteristics of the system of Fig. 1;

Fig. 3 is diagram for facilitating the description; and,

Fig. 4 illustrates a modification of the system of Figure 1.

45 The relay system of Fig. 1 comprises a thermionic valve 11 of the type known as a power pentode and known commercially as a 247. The pentode has five concentrically spaced electrodes which are respectively designated as fila- 50 ment or cathode 13, control grid 14, screening grid 15, cathode grid 16 and plate or anode 1'1. The system is powered by a transformer 20 having a primary winding 21 connected to a suitable alternating current supply. A secondary winding 55 portion 22 supplies power to heat the cathode 13;

another secondary winding portion 23 supplies the cell voltage or grid bias; and still another winding portion furnishes the plate voltage. A potentiometer 21 connected across the winding portion 23 has its adjustable voltage arm con- 5 nected to a cathode 3| of a photoelectric tube or control means 30, an anode 32 of which tube is connected to the control grid [4 of the thermionic valve 1 I. No other connections are made to either the anode 32 of the tube or to the control grid 14 of the valve. The winding 24 is connected directly to the screening grid and is connected through a current responsive device to the plate or anode 11. The current responsive device preferably comprises a relay 36 shunted by a con- 15 denser 3'1.

While the system is powered with alternating current, the valve II is inherently self rectifying so that the current in its plate circuit has a direct current component flowing, in the conventional sense from the plate 11 to the cathode 13 within the valve itself. Likewise the photo tube 30 is self rectifying and is traversed by a direct current flowing from its anode to its cathode.

In operation the current in the plate circuit of 25 the pentode decreases as the illumination of the tube increases and/ or as the negative tube voltage is increased by means of the potentiometer 21. The tube voltage is taken as being negative with respect to the cathode I3 of the valve, that 30 is, with respect to the center tap of the winding 22, considering the plate voltage to be positive with respect thereto. The potentiometer 21 may therefore be adjusted to accommodate the system to difierent conditions of ground or base illumination on the tube 30.

Thus, assuming that our relay system as shown in Fig. 1, is to be used in a burglar alarm or the like wherein the system is to be responsive to an interruption of alight beam, the grid bias voltage should first be reduced by means of the potentiometer 21 until the current through the relay 36 is sufiicient to pick up the relay armature when the light beam is interrupted. When the light beam is again permitted to fall upon the cathode 31 of the photo tube, the current through the relay 36 will decrease a certain amount. If this amount is insufficient to cause the relay 36 to drop its armature, the light beam must be adjusted to direct more light into the tube. Should the systern be required to respond to an interruption of the light beam regardless of changes in background illumination, that is, extraneous light reaching the tube, the potentiometer adjustment must be made when the background illumination is at a maximum, and the light beam must be bright enough to reduce the plate current through the relay 36 to the required value when the background illumination assumes its minimum value.

With the above adjustments properly made, the armature of relay 36 will lie in its normal or released position when the photoelectric tube is illuminated by the light beam. When the light beam is interrupted due to a person or some other object passing through it the relay 36 will immediately draw up its armature. When the beam again falls on the tube, the armature will release. Obviously, the armature of relay 36 may be made to operate contacts or the like which may be utilized for control purposes in any known manner.

In Fig. 2, we have shown graphically the manner in which the plate current varies with illumination on the photoelectric tube 32 at various tube Voltages or grid bias potentials. Cell illumination is plotted linearly along the abscissa and the unidirectional component of the current to the plate or anode I1 is plotted along the ordinate. Curves are given for several constant values of tube voltage or grid bias, which values are noted in root-mean-square values.

The theoretical explanation of the operation of our system is believed to be as follows: Referring to Fig. 3, the inherent mutual capacities of certain of the parts are indicated by condensers drawn in dotted lines and connected between those parts. Because of the presence of these capacities there exists an impedance bridge circuit extending from the high potential screening grid 55 to the control grid and the anode 32 of the photo tube connected thereto, and extending thence to the cathode 3| of the tube, the cathode 3! being maintained at a relatively low voltage by its connection to the variable tap of the potentiometer 27. This impedance bridge circuit being traversed by alternating current exercises some control over the potential of the grid 14. The capacity of the grid M to the filament l3 may also affect the control grid potential, but the capacity of the plate I! to the control grid [4 probably has little effect on the system because of the shielding action of the cathode grid l6. It is believed that the control exercised over the grid I l by the various capacities making up this impedance bridge serves to stabilize the action of the valve to prevent blocking or floating.

The valve being a rectifying device, it passes current through its plate circuit during only onehalf of the voltage cycle, that half being the one which makes the plate I! positive with respect to the cathode I 3. During that half cycle, the oathode 3! of the photo tube is made negative with respect to the cathode 13 of the valve and also with respect to the screening grid I5 and plate I! thereof. As a result the cathode 3| is also negative with respect to the anode 32.

When light falls on the tube, a direct current flows, in the conventional sense, from the control grid l4 to the photoelectric tube. That is, electrons flow from the anode 32 of the tube to the control grid of the valve. It is to be noted that the flow of electrons into the valve via the grid is contrary to what has heretofore usually been observed in valve operation. The electrons probably escape from the grid as a result of secondary emission and/or gas ionic flow. We have found that a small amount of gas improves a valve for use in our system and in general there will be a certain optimum condition at which the system will operate best. Electrons are also emitted from the cathode l3 to the several other electrodes of the valve including the grid l4. These various electron streams constitute a direct current bridge circuit which exercises a further control of the control grid potential and therefore of the current flowing in the plate circuit of the valve.

When the photo tube is dark its conducting impedance is high and the potential of the control grid it is determined largely by conditions within the valve itself such as ionic flow and capacity eifects. When light falls on the photoelectric tube SS, electrons flow therethrough to the control grid and change the conditions of electronic and/or ionic flow within the Valve so as to lower the potential of the control grid I4 relative to the cathode l3, assuming that the anode I! is positive with respect thereto. Or to put it differently, the photo tube lowers its impedance when illuminated, and thereby brings the potential of the control grid Hi nearer to the low potential of the cathode 3| of the photo tube. When the potential of the control grid is lowered, the plate current decreases in accordance with the usual operating characteristics of electron valves.

We have found the present system to be more or less responsive to changes in the capacity conditions of the circuit and that small capacities may be introduced to supplement the potentiometer 2i in compensating for background illumination and for adjusting the plate current to the desired value when the photo tube is dark. In Fig. 4 we show one manner of incorporating such a capacity in the circuit of Fig. l. A variable condenser il having a capacity of the order of ten micro-microfarads, is shunted across the terminals of the photoelectric tube 33. The circult is otherwise the same as that shown in Fig. 1. Increasing the capacity of the condenser 40 serves to lower the current through the relay 36. The condenser ill 'may also be employed to counteract the effect of a high capacity to ground (cathode potential) in the plate circuit of the valve.

We have found that the system of our present invention is-extremely sensitive to small variations of light, and especially so at low light levels. The system is therefore ideally suited for use in burglar alarm systems and the like.

We have also found that a four electrode or screen grid valve may be employed in the present system instead of the pentode, and althoughits power output is limited, the four electrode valve is to be preferred under certain conditions when large power outputs are not required, as, for example, when the system is used to excite other electronic equipment such as a subsequent stage of amplification, or a controlled glow valve.

It will be apparent that'the application of the present invention is not necessarily confined to purely electronic devices. Under certain conditions, it may be desirable to employ gaseous ionic valves, preferably of the non-glowing type in place of the pentode. Furthermore, suitable impedance devices other than a photoelectric tube may be employed, which devices may or may not be responsive to light.

It will be apparent to those skilled in the art that the herein disclosed embodiment is by way of illustration only. Wetherefore do not wish to be limited except by the scope of the appended claims.

We claim:

1. In a photoelectric system, an electronic valve having a control grid and at least three other electrodes, a photoelectric tube having an anode and a cathode, said anode of said tube being connected to said control grid of said valve, there being no other connection to said grid, and means connecting said cathode of said tube to at least one of said other electrodes of said valve.

2. In combination, a photoelectric tube having a cathode and an anode, an electronic valve having a cathode, an anode, a control grid and a screening grid, the anode of said photoelectric tube being connected to said control grid, and means for supplying alternating voltages to said system for its operation so that the voltage applied to said screening grid relative to the cathode of said electron valve is of a sign opposite to the sign of the voltage applied to the cathode of said photoelectric tube relative to said screening grid, said photoelectric tube constituting the sole conducting connection between said control grid and any other electrode of said valve.

3. In a photoelectric system, an ionic valve having a cathode, a control grid, a screen grid and an anode, a photoelectric tube having an anode connected to said control grid, a source of alternating voltage connected to another electrode of said tube and to said cathode, screen grid and/or anode of said valve, said tube constituting the sole conducting connection of said control grid with said source of voltage.

4. In a photoelectric relay system, a pentode valve including a cathode, a control grid, a screening grid, a cathode grid and an anode, said cathode grid being connected to said cathode, a photoelectric tube having an anode connected to the control grid of said valve, said tube also having a cathode, current responsive means connected to the anode of said valve, alternating current power supply means connected to said current responsive means, to the screening grid of said valve, to the cathode of said valve and to the cathode of said photoelectric tube so as to supply voltages tosaid current responsive means and said screening grid which are of similar sign with respect tothe cathode of said valve and to supply a voltage to the cathode of said tube which is of a sign with respect to the cathode of said valve opposite that supplied to said screening grid, said photoelectric tube constituting the sole conducting connection between the control grid and cathode of said valve.

5. In a photoelectric system, a pentode valve having a cathode and a control electrode, means for supplying alternating voltages to the system for its operation, a photoelectric tube connected in a circuit between said cathode and control electrode so as to pass electrons through said tube to said control electrode, said tube being responsive to illumination to control said valve, and a capacity circuit also connected between said cathode and said control electrode to also control said valve, said photoelectric tube constituting the sole conducting connection between said cathode and control electrode of said valve.

6. In a photoelectric system, a pentode valve having a cathode and a control electrode and a control circuit connected between said cathode and control electrode, said control circuit comprising all the connections to said control electrode and including a photoelectric tube, said control circuit being so constituted as to pass electrons therethrough only from cathode to grid of said valve.

7. In a photoelectric system, an electronic valve having a control grid and at least three other electrodes, a photoelectric tube having an anode and a cathode, said anode of said photoelectric tube being connected to said control grid of said valve, there being no other connection to said grid and means connected to the cathode of said photoelectric tube and to the said other electrodes of said electronic valve for supplying alternating voltages thereto for the operation of the system.

8. In a photoelectric system a pentode valve having a cathode and a control electrode, a control circuit connected to said control electrode and means for supplying alternating voltages to said valve and control circuit for the operation of the system, said control circuit comprising all the connections to said control electrode and including a photoelectric tube, said circuit being so constituted as to pass electrons therethrough only from cathode to grid of said valve.

ARCHIE J. MCMASTER. JAMES G. WELLS. 

